Sensitive photodetectors can be easily damaged through short, high intensity light pulses, which are above the specified maximum intensity of the photo detector, thus presenting an optical overload condition. Often the effects of such damage become apparent through device ma1functions, which could occur months or even years later. The lack of proof that an overload ever occurred can lead to disputes between OEMs and customers since it is not possible to rule out the possibility of an overload in failed equipment. Generally, it is problematic that it is impossible to tell that reliable detector operation may have been compromised.
The avalanche photo diode (APD) exhibits gain values up to 100 and is therefore able to detect levels at −35 dBm (=0.3 μW). However, until recently PIN diodes have been the preferred photo detector in networking applications because of their low noise sensitivity, despite requiring a preamplifier if acceptable small signal performance is to be achieved. Through ever improving APD small signal performance, even the need for an optical amplifier may be removed, leading to a dramatic reduction in the cost of optical networking equipment.
Hence, now that APDs having improved signal to noise ratios have been developed, they are becoming preferred to the PIN diode with preamp combination. Devices integrated together with output conditioning amplifiers and thermisters for temperature compensation are available.
In use, the APD must be fed from a source of supply voltage from a control circuit which is able to supply a stable supply voltage. Moreover, because the avalanche mechanism within the device is dependent upon its reverse bias voltage, the actual voltage across the device must be kept substantially constant for accurate input level sensing and must be additionally regulated to compensate for temperature effects since the avalanche mechanism is temperature dependent also.
APDs can be damaged or destroyed through high current flow caused by strong incident light. The duration of this type of damaging optical overload condition can be as short as microseconds. In the case of a short over load condition, the APD might well be able to continue operation once normal conditions have been restored; however, it's potentially damaged internal structure remains a reliability issue.
Since an APD intended for optical networking equipment is a high value component in itself and moreover may be very expensive to replace in the field, it is necessary to have some protection built into detection circuitry.
In the prior art a common solution is to place a high impedance resistor in series with the APD (as may be seen for example in FIG. 1).
In FIG. 1, there is shown the supply portion of a typical control circuit for an APD photodetector 10. The photodetector 10 is fabricated as part of an integrated device 12, together with an output amplifier 11. The integrated device, 12 will typically include a thermister (not shown) to sense the temperature of the device, such that temperature variations may be compensated by a further portion of the circuit, which is not shown in the figure. Detector 10 receives input light from a fiberoptic 16. The detector is supplied with a voltage from power amplifier 14 which is stabilized by a feedback resistor network Rf, Rg. Since there are typically wide variations in individual APD performances, the equipment is typically individually calibrated and one way to do this is to drive the power amplifier by a digital to analog converter 15, which may be set to give the required multiplication factor from the individual diode present in the integrated package 12 by controlling the actual reverse bias voltage applied. Photodiode 10 is protected by the inclusion of a series resistor Rs.
The operating current of the APD is typically between 1 and IOμA. In the case of a 100 k series resistor, the voltage drop across this resistor might be approximately for example 0.3V. During an optical overload condition however, the current can ramp up to 0.5 mA for a short period, causing the voltage across Rs to rise to several tens of volts, reducing the bias voltage across the APD and minimizing its power dissipation and thus affording some protection to the detector 10. Once the overload condition disappears, the current would return to its normal operating value.
Although offering protection, this arrangement itself has some drawbacks. It is not possible from the circuit to tell that an overload event has occurred and obviously not its magnitude or duration. This being the case, the reliability issue of a potentially damaged device remaining in service is still present.
Moreover, the protection is provided at the expense of circuit performance and complexity. Since the gain factor (M) depends on the reverse bias voltage of the APD, the voltage drop across the series resistor, Rs, will itself cause the actual gain factor to differ from the intended value. Even quite small variations in bias can cause significant amplification factor variations and the higher value of Rs the greater will be the voltage drop affecting the amplification delivered. Unfortunately, in order to achieve adequate protection, the value of Rs has to be sufficiently high that the variation in amplification factor caused by the voltage drop across it requires compensation of the detector output, requiring that DAC 15 be driven hard.